Bicycle is a tool to help the people to move faster by wheels and is an important turning point in the history of vehicles to improve the efficiency of "walking". From the original multi-wheel to the present double-wheel, the revolution of driving mechanism is the most important change in the improvement of performance. From the original direct pedaling to directly drive the driving wheel, through the chain drive system to turn the chain wheel and the rear hub sprocket, to the recent multi-gear type "variable speed drive", improvement on driving mechanism has been continuously pushing out. Therefore, cycle chain is acting a very important role in the history of the revolution of bicycles. Recently, there is another design of rack mechanism link motion used for the driving mechanism of a bicycle. However, this design is not practical, because it requires repeatedly the operation of "vertical pedaling" with the feet which is not in conformity with physical dynamics. Therefore, this design can not enjoy general popularity.
As we all know, the chain drive bicycles may still have some drawbacks remain unsettled as described hereunder.
1. Space occupation: The cycle chain which is to engage with the chain wheel and the rear hub sprocket of the driving wheel takes a big space to install at one lateral side. Therefore, the gravity of the bicycle is inclined toward the side where the cycle chain is engaged and, the outer appearance of the bicycle is not balanceably designed.
2. Less security: Since the cycle chain is to engage with the gear wheels, while running at higher speed, the chain links are repeatedly driven to leave from one tooth to engage with the other. While riding, the bottom of the rider's pants may be easy to swing and to thrust into the gear to be engaged with the cycle chain to abruptly stop the driving and to further cause the rider to get hurt in a fall. It is believed that every bicycle rider has experienced feet being stained or hurt by the cycle chain while riding
3. Complicated structure: In order to protect the chain from exposing, a chain guard is required. And in order to fix the chain guard fastening means must be provided. Therefore, the chain driving mechanism requires relatively more complicated structure which becomes more difficult to operate, assemble and maintain, and is easy to break down.
4. Chain dropping: Looseness of the tension of cycle chain, or displacement of engaging position due to excessive driving speed, or thrust of external substances may cause the chain to drop to further cause the bicycle lose control or capsize. Since the body of the bicycle is very light, it is very dangerous while riding at high speed.
5. Since the cycle chain is exposed outside, it is easily damaged or the lubrication is easily dried to deteriorate the perfomance of the cycle chain. Therefore, constant lubricating is required In case the chain wheel or the rear hub sprocket is damaged or offset, stable engagement will be unable to achieve. Further, the tension of the chain must be kept within a limited range to let the chain work properly. Attaching a chain guard to entirely enclose the cycle chain to prevent of the former problem from happening (as used in earlier traditional bicycles), it will increase the cost, complicate the assembly procedures, and cause chain trouble (cycle chain scrapes the chain guard).